The transient behavior of a polymer exchange membrane fuel cell (PEMFC) with dry gases fed in is investigated in this paper. Periodical ramp sweeps were applied to the fuel cell. When dry gases are fed into the fuel cell, the drying out effect of the polymer electrolyte is measured during the sweep process. The polarization curve of the fuel cell and time-dependent membrane resistance is shown. A hysteresis effect of the fuel cell polarization curve is found. The "threshold", defined as the match point of the forward and backward sweep current-voltage curve is introduced. In this paper, threshold indicates the status of the fuel cell membrane hydration. The transient behavior during load change is investigated when dry gases are fed into the fuel cell. The comparison between the ramp sweep and the step sweep experiments shows that the fuel cell could run stable at the threshold current density under a specific operating condition. (c) 2006 The Electrochemical Society.